Monitoring respiratory function plays an important role in the management of patients in clinical settings. A change to respiratory gas composition is one aspect of respiratory function that can be used to assess patient health and respiratory status. The concentrations of gases in a patient's respiratory system are the result of ventilation, gas exchange, gas transport, and the control of ventilation (internal or external). Some examples of respiratory gases monitored include oxygen (O2), carbon dioxide (CO2), nitrous oxide (N2O), and nitrogen (N2).
For example, the concentration of CO2 expelled from the lungs during expiration may be the result of ventilation, perfusion, metabolism, and their interactions. Changes in CO2 concentration reflect changes in any or all of these factors. It is therefore vital to monitor and measure the concentration of CO2 to provide a snapshot of a patient's respiratory status. It provides immediate information on CO2 production, ventilation perfusion (V/Q) status, and elimination of CO2 from the lungs.
Capnography is the measurement of exhaled CO2. This is the CO2 that has diffused out of the blood into the alveolar air and is subsequently exhaled. End tidal CO2 (ETCO2) is the partial pressure or maximal concentration of CO2 at the end of an exhaled breath and is an important metric used to non-invasively estimate the patient's arterial CO2 concentration (PaCO2). The difference between ETCO2 and PaCO2 is usually between 2 and 5 mmHg. Therefore, ETCO2 is a good representation of PaCO2. However, any conditions causing ventilation perfusion mismatch will cause ETCO2 to underestimate PaCO2.